Encounters with hydrogen sulfide occur frequently during drilling operations for oil and gas. Occasionally, during the course of drilling through subterranean formations, a drill will puncture a strata with high hydrogen sulfide gas content. Such encounters are known in the industry as sour gas kicks. As the drilling fluid is circulated through the drill string and drill bit penetrating such subterranean formations to the bottom of the borehole and then back to the surface via the annular space between the drill pipe and bore wall, the drilling fluid will entrain high levels of hydrogen sulfide which subsequently are released at the surface. Hydrogen sulfide emerging from such kicks present severe hazards to both drilling personnel and equipment.
Prolonged exposure of drilling personnel to low concentrations of hydrogen sulfide can be lethal, and exposure to higher concentrations can be fatal within minutes. Exposure of drilling equipment to hydrogen sulfide can cause equipment failure due to sulfide stress cracking and hydrogen embrittlement of steel. Small concentrations of hydrogen sulfide as low as 0.1 ppm can greatly reduce equipment life. Hydrogen sulfide can react with metallic and dissolved iron to form insoluble iron sulfides subsequently causing well and equipment fouling problems. Hydrogen sulfide can alter drilling fluid pH consequently causing dramatic changes in drilling fluid viscosity. Hydrogen sulfide is also an environmental pollutant giving off a nauseating "rotten egg" odor. Hydrogen sulfide is not only toxic and corrosive but is highly flammable.
Zinc carbonate has been widely used in the industry to scavenge hydrogen sulfide in drilling fluids. However, zinc compounds react with sulfide ions via an acid-base ionic reaction; and for the reaction to proceed, the hydrogen sulfide must be continually neutralized by alkaline zinc carbonate. Therefore, at a low pH, zinc carbonate would not provide adequate scavenging protection. In addition, zinc compounds are relatively expensive and if used in excess may cause drilling fluid rheology problems.
Iron oxides have been effectively employed to scavenge hydrogen sulfide as disclosed in U.S. Pat. Nos. 4,008,775 and 4,324,398. However, iron oxides are difficult to disperse and impart additional weighting properties that increase the density of the drilling fluid. Additionally, iron oxides are dependent upon a low pH to effectively scavenge hydrogen sulfide.
Because the foregoing hydrogen sulfide scavengers are insoluble, they may need additional dispersing agents as disclosed in U.S. Pat. No. 3,928,211. Moreover, such insoluble scavengers would be relatively slow acting, i.e., needing to be fully dispersed before complete reactions could proceed. It would, therefore, be desirable to provide a hydrogen sulfide scavenger that is easily dispersed, fast acting and which provides excellent scavenging power without adversely altering the drilling fluid rheology.